In an ordinary optical disk such as CD or CD-ROM (to be referred to as “CD” hereinafter), embossed pits corresponding to recording data are formed on one side of a 1.2 mm-thick transparent substrate and a reflection film made from aluminum (Al) et al. is formed on the substrate. Information recorded on this CD is reproduced by applying converged beams from the side opposite to the reflection film side of the transparent substrate.
In contrast to this, in a DVD or DVD-ROM disk (to be referred to as “DVD” hereinafter) having a higher recording density than the above CD, finer embossed pits than those of the CD are formed on one side of a 0.6 mm-thick transparent substrate and a reflection film made from Al et al. is formed on the substrate. Information recorded on the recording side of the DVD is reproduced by applying converged beams from the side opposite to the reflection film side of the transparent substrate like the CD.
PC (polycarbonate) which is a transparent resin material is generally used as the material of the 0.6 mm-thick substrate. Since a 0.6 mm-thick PC substrate does not have satisfactory mechanical properties and is warped as it is, two 0.6 mm-thick PC substrates are assembled together in such a manner that their recording surfaces are adjacent to each other to form a 1.2 mm-thick disk having satisfactory mechanical properties.
The reason that the thickness of the DVD substrate is 0.6 mm is that a tilt margin is ensured. If the track pitch becomes narrower and the pit density becomes higher, the inclination of the disk so called “tilt margin” will be reduced. By reducing the thickness of the substrate from 1.2 mm to 0.6 mm, a tilt margin can be ensured but a reduction in mechanical strength cannot be avoided.
To reduce the thickness of the substrate and ensure mechanical strength at the same time, it is proposed that the center portion of a disk is made thick (JP-A 9-204686) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, to ensure mechanical strength, the limit thickness of the substrate of a signal recording area has been 0.6 mm. Although it is also reported that the thickness of a transparent substrate is set to 0.1 mm to 0.6 mm (JP-A 9-204688), the thickness of a protective substrate for holding a recording film and the thickness of a reflection film are not mentioned, thereby making it impossible to implement a disk.
However, in recent years, various new optical recording media have been proposed. For example, U.S. Pat. No. 5,972,459 (JP-A 11-7658) proposes an optical recording medium which comprises a substrate made from a thermoplastic resin and having a thickness of 0.3 to 1.2 mm, guide grooves on the above substrate, and a recording area consisting of at least a reflection film and a phase change recording film on the guide grooves in the mentioned order, a transparent protective film having a thickness of 3 to 177 μm being formed in at least the recording area, and which satisfies the following relationship:    Δt≦5.26 (λ/N.A.4) (μm)wherein Δt is the thickness variation of the transparent protective layer, N.A. is the numerical aperture of an optical system for reproduction or recording/reproduction, and λ is the wavelength of the optical system.There is provided an optical recording medium whose capacity can be increased by specifying the relationship between the thickness of the transparent protective layer and the thickness variation of the layer.
Conditions required for an optical recording medium having a high recording density of 8 GB as one example of the above optical recording medium are obtained as follows. That is, the recording/reproduction optical system must satisfy λ≦0.68 μm and N.A./λ≦1.20, the thickness t of the transparent protective layer in the recording area must be 3 to 177 μm, and the thickness variation of the transparent protective layer must satisfy the following expression.    Δt≦±5.26 (λ/N.A.4) (μm)    Track pitch P≦0.64 (μm)    Tolerance ΔP≦±0.04P (μm)    Linear density d≦0.1161/P (μm/bit)    Disk skew Θ≦84.115×(λ/N.A.3/t)    Eccentricity E≦67.57P (μm)    Surface roughness Ra≦±3λ/100 (within spot exposure area).
U.S. Pat. No. 6,159,572 (JP-A 11-296904) proposes as a one-sided recording system “an information recording medium which comprises a substrate having a recording face provided with embossed pits or guide grooves, a reflection film formed on the above recording face of the substrate, and a protective film formed on the reflection film, whose faces consist of a first face having the protective film formed thereon and a second face opposite to the first face, and which reproduces recorded information based on a change in the intensity of reflected light by applying a light beam from the first face side, wherein
the distance between the recording face of the substrate and the first face is smaller than the thickness of the substrate and the first face is smooth.”
The above publication also proposes as a both-sided recording system “an information recording medium which comprises a substrate having opposite first and second recording faces provided with embossed pits or guide grooves, first and second reflection films formed on the first and second recording faces of the substrate, and first and second protective films formed on the first and second reflection films, respectively, whose faces consist of the first face having the first protective film formed thereon and the second face having the second protective film formed thereon, and which reproduces recorded information based on a change in the intensity of reflected light by applying a light beam from the first face side and the second face side, wherein
the distance between the first recording face of the substrate and the first face and the distance between the second recording face of the substrate and the second face is 5λ/(4n) to 0.6 mm (λ is the wavelength of the light beam, and n is the refractive index of the first or second protective film of light having the above wavelength λ), the thickness of the substrate is 0.6 mm to 1.2 mm, and the distance between the first face and the second face is 1.2 mm or less.”
The above publication is aimed to provide an information recording medium which ensures a sufficiently large tilt margin and mechanical strength even when its recording density is increased.
In these proposed recording media, the material of the substrate is not taken into special account. For example, U.S. Pat. No. 6,159,572 enumerates acrylic resin such as polymethyl methacrylate (PMMA), polycarbonate resin, epoxy resin, styrene resin, glass, metals such as Al, alloys and ceramics as the materials of the substrate.
However, in the constitution of the above recording medium, it is difficult to avoid the occurrence of skew for the following reasons (1) to (4).    (1) at the time of injection: generation of stress by shearing stress when a resin flows in a cavity (molecular orientation distortion)    (2) Completion of filling: when a resin is filled in a cavity, a flow of the resin is abruptly stopped by the sudden stop of the movement of a screw, whereby the inertia forces of the resin and the screw are all applied to the substrate    (3) Pressure retention: Since pressure is applied to the resin for the prevention of a backflow of the resin and the prevention of sink caused by volume shrinkage until the gate is sealed at the time of injection, a pressure distribution occurs in the whole substrate.    (4) Cooling: stress is generated by thermal shrinkage according to a temperature distribution.
Therefore, to improve the skew, JP-A 11-242829 proposes “an optical recording medium which comprises at least a recording layer and a transparent protective layer on a substrate in the mentioned order, for recording and/or reproducing an information signal by applying light from the transparent protective layer side, wherein
the substrate comprises a first resin layer for forming a face on which the recording layer is formed and a second resin layer which is formed on the first resin layer and made from a resin material having a higher flexural modulus than the resin material forming the first resin layer.”
Meanwhile, even when the problem with mechanical properties is solved by the above improvement, in an optical recording medium for recording a signal on one side, the deformation of a disk is caused by moisture absorption due to environmental temperature and humidity variations.
In the case of a DVD, a polycarbonate substrate having a water absorption of 0.3% or more is used but two 0.6 mm disks are assembled together in such a manner that their signal recording sides become adjacent to each other. Therefore, even if the water absorptions of the disks are high, water absorption is well balanced and deformation hardly occurs. However, since a high-density disk having high N.A. (numerical aperture) has a signal recording layer on one surface layer, water absorption is imbalanced, thereby causing water absorption deformation. Particularly, during driving, a focus error readily occurs such that a signal cannot be read due to the deformation of a disk because a drastic change easily occurs from a high temperature and a low humidity in the disk drive.
To suppress such water absorption deformation, U.S. Pat. No. 6,201,783 (JP-A 2000-11449) proposes “a disk-like information recording medium which comprises a substrate, a recording layer, formed on the substrate, for recording an information signal, and a transparent protective layer laminated on the recording layer and which records and reproduces an information signal by applying light from the transparent protective layer side, wherein
the substrate comprises a resin core layer and resin surface layers integrated with the core layer, having irregularities for information signals on the recording layer on one side and having fluidity compared with the core layer. A resin having a water absorption of 0.3% or less is used as the surface layer resin of the substrate. It is proposed to solve the above problem with the complicated structure of the substrate formed by two-color molding or sandwich molding.
Problems to be Solved by the Invention
In view of the trend of the above-described prior art, the inventors of the present invention have conducted intensive studies to provide a substrate for an optical recording medium of the above system having a simple structure without making it complicated in structure and have attained the present invention.
Means for Solving the Problems
According to researches conducted by the inventors of the present invention, the above object of the present invention is attained by an optical recording medium which comprises a 0.3 to 2.0 mm-thick substrate made from a thermoplastic resin and having embossed pits or guide grooves, a reflection layer formed on the substrate and a 3 to 200 μm-thick transparent protective layer formed on the reflection layer and which reproduces recorded information based on a change in the intensity of reflected light by applying a light beam from the surface side of the transparent protective layer, wherein
the substrate is formed from (i) a thermoplastic resin which comprises at least 50 wt % of an aromatic polycarbonate resin obtained by polymerizing a dihydroxy compound containing at least one aromatic dihydroxy compound selected from the group consisting of 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane and 9,9-bis(4-hydroxy-3-methylphenyl)fluorene in an amount of at least 20 mol % based on the total of all the dihydroxy compounds as a bisphenol component; or
more generally, the substrate is formed from (ii) a thermoplastic resin having the following properties (1) to (4):    (1) a flexural modulus of 24,000 kgf/cm2 or more,    (2) a saturation water absorption of 0.33 wt % or less,    (3) a deflection temperature under load of 125° C. or higher, and    (4) a loss tangent (tan δ) of 0.006 or more.
The optical recording medium of the present invention records and reproduces a recorded information by applying a light beam from the surface side of the transparent protective layer having a thickness of 3 to 200 μm. This optical recording medium can record and reproduce a large volume of information and can be used as an optical recording medium using a short wavelength laser (for example, blue purple semiconductor laser having a wavelength of 405 nm). This optical recording medium is called “Blue-ray Disc” to distinguish it from conventional CD's and DVD's.